1. Field of the Invention
The present invention relates to a photosensitive member for electrophotography and more particularly to a photosensitive member for electrophotography having no fatigue effect.
2. Description of the Prior Art
Various types of photosensitive members for electrophotography are known and in accordance with the electrophotographic process to be employed, a most suitable type of photosensitive member is selected among them. A photosensitive member of the type which has an insulating layer on its top surface is used to form an electrostatic latent image on the insulating layer. For such a type of photosensitive member, it is required to inject an amount of charge into the interface between the insulating layer and the photoconductive layer by charging. As an example of the electrophotographic process for which such a type of photosensitive member is suitably used, mention may be made of a process comprising the steps of a primary charging, an imagewise exposure, an AC discharging simultaneous with or after the imagewise exposure or a secondary charging with a polarity opposite to that of the primary charging and a whole surface exposure. In case that the photoconductive layer is composed of a p-type semiconductor such as Se, SeTe or the like, the primary charging is effected by corona discharging with negative polarity and a certain amount of positive charge is injected into the photoconductive layer through its support so that the charge may be migrated into the interface between the insulating layer and the photoconductive layer under the effect of an electric field applied on the latter. When it is difficult to inject the charge through a support, an alternative method can be employed according to which the photosensitive member is uniformly exposed to a light just before or simultaneously with the corona discharge with negative polarity so that a suitable amount of positive charge may be present in the interface between the insulating layer and the photoconductive layer. If this exposure to light is carried out from the side of the support, the support must be of light transmissive material such as Nesa glass, resin film and the like. In case that the photoconductive layer is composed of a n-type semiconductor, the polarity of charging will be positive and the charge migrated into the interface will be negative. Such an injection of a suitable amount of charge between the insulating layer and the photoconductive layer is absolutely necessary to produce an electrostatic image having a high electrostatic contrast. To this end, if the electric conductive support is made of metal, for example, an attempt has been made to provide a charge injection layer between the support and the photoconductive layer as disclosed in Japanese Patent Publication No. 6223/1974. The charge injection layer serves to supply an adequate amount of charge into the photoconductive layer when charging is effected, and the charge thus supplied contributes to making a suitable amount of charge present between the insulating layer and the photoconductive layer. However, it has been found that even when such a charge injection layer is provided, a further improvement should be made.
That is, when the photosensitive member should be repeatedly used and when the cycle of the repeated use is increased up in the speed, it is often observed that the amount of charge existing in the interface between the insulating layer and the photoconductive layer is gradually decreased and as a result the contrast of an image formed after many times of repeated use is remarkedly reduced. This disadvantageous phenomenon may be explained as a fatigue effect of the photosensitive member.